The present invention relates to a thick film inorganic electroluminescent lamp and method of construction thereof.
Electroluminescent lamps have a number of performance parameters, including brightness, efficiency and life. While any one parameter can be increased, for example brightness, other parameters must usually be reduced, such as lamp life or efficiency.
Electroluminescent lamps are constructed as a lossy capacitor, generally having a dielectric material between two electrodes. A light-emitting layer having phosphor particles is also located between the electrodes, either within the dielectric layer or as a separate layer between the electrodes. Typically one of the electrodes is transparent to allow light generated by the light emitting layer to escape, and thus the lamp emits light. The transparent electrode is typically a material such as indium tin oxide.
To manufacture an electroluminescent lamp, each of the layers may be provided in the form of an ink. The inks, which may be applied by screen printing or roll coating include a binder, a solvent, and a filler, wherein the filler determines the nature of the printed layer.
A typical solvent is dimethylacetamide (DMAC) or ethylbutylacetate (EB acetate). The binder may be a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy or Kynar 9301, a proprietary terpolymer sold by Atofina, dissolved in N,N Dimethylacetamide. Other binders used include ShinEtsu's CR—S (with or without Cr—U) dissolved in N,N dimethylformamide.
The light emitting layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide phosphor particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and barium titanate (BaTiO.sub.3) particles. A rear (opaque) electrode may be screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon.
When such a lamp is used in portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, power needs to be provided by an inverter that converts low voltage, direct current into high voltage, alternating current. In order for a lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is usually necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor particles. The frequency of the alternating current through an electroluminescent lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz, leading to premature failure. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.